1. Field of the Invention
The present invention relates generally to the field of four-wheel drive power transmission systems for vehicles, particularly to such systems that transmit power to the first or two sets of wheels while the wheel sets have the same speed, and to the other set of wheels when the speed of the first wheel set exceeds that of the other wheel set.
2. Description of the Prior Art
Drive mechanisms have been devised that transmit power to only one set of wheels when the four wheels of the vehicle are rotating at nearly the same speed. However, when the driven wheels begin to overrun, the nondriven wheels are drivably connected to the transmission and become driven, whereas the driving wheels in this condition are no longer supplied with power. Sometimes this action is done by providing the nondriven set of wheels with an overrunning clutch and by gearing the clutch such that it becomes engaged when a predetermined speed differential between the driving and nondriven wheels occurs. This requires that the overunning clutch be manually locked in order to obtain four-wheel drive when the vehicle is driven in reverse or when engine braking is required. Other systems use a conventional differential to allow the front and rear axelshafts to deliver power while rotating at different speeds. These systems generally require a manual lock-up device to prevent excessive wheel spin when encountering surfaces on which the wheels may slip. It is desirable in the operation of a four-wheel drive vehicle that the driven wheels should be driveably disconnected from the transmission when the wheels are not driven in order to reduce drag on the engine and to avoid unnecessary wear.
In four-wheel drive systems, power is transmitted to a set of driven wheels and power is transmitted to another set of wheels only when the first set spins relative to the road surface. Usually it accomplishes this result by providing gearing between the driven and nondriven axleshafts that permits an element of a roller clutch in the driveline to overrun unless this slippage occurs. This overrun results because the gear set has a gear ratio that permits one element of the clutch to rotate at a preselected different speed than that of the nondriven wheels. When slippage of the driven wheels occurs, this speed differential is overcome and power is transmitted to the usually nondriven wheels. It is possible in systems of this type, after a forwardly driven vehicle is stopped and then driven in reverse, that the rear wheels will transmit power through the overrunning clutch and the gear set to the driven wheels. When this occurs, the speed of the driven wheels tends to exceed the speed of the usually nondriven wheels because of the effect of the gear ratio of the gear set that connects the axle. But because the wheels are of the same size and their linear speeds must be the same in straight ahead driving, the driven wheels are forced to scrub and slide across the road.